Currently, in the field of displays for use in AV equipment including a home-use television system and OA equipment such as personal computers, there is a growing demand for light-weight, thin, low power consuming, high-definition, and large displays. In order to meet the demand, development of display devices such as CRT, liquid crystal display device (LCD), plasma display device (PDP), EL (electroluminescent) display device and LED (light emitting diode) display device has been actively carried toward realizing large displays and practical applications.
In particular, liquid crystal display devices can realize an extremely small thickness (depth), less power consumption, and full-color display easily compared to other types of display devices. Therefore, the liquid crystal display devices have been used in various fields in resent years, and development of a large display is greatly expected.
However, if the screen size of the liquid crystal display device is increased, the fraction defective due to disconnection of signal lines, pixel defects and the like abruptly increases in the fabrication processes. As a result, the price of the liquid crystal display device increases.
In order to solve such a problem, a technique for realizing a large display is disclosed in Japanese Publication for Unexamined Patent Application Nos. 213621/1989 (Tokukaihei 1-213621) and 127605/1993 (Tokukaihei 5-127505), and Japanese Publication for Examined Utility Model Application No. 17178/1994 (Jitsukohei 6-17178). In the disclosed technique, a large display is realized by fabricating one liquid crystal display device by joining a plurality of liquid crystal display panels. This technique is generally called a multi-panel technique.
However, when the liquid crystal display device of multi-panel type is fabricated by simply joining liquid crystal display panels together, light from a backlight leaks through gaps produced in the joints of the liquid crystal display panels. Thus, the joints of the liquid crystal display panels are noticeable.
In order to produce a natural image on such a large display, a technique for rendering the joints in the display screen less noticeable is required. Then, a liquid crystal display device of multi-panel type having less noticeable joints was proposed. The following description will discuss this liquid crystal display device.
FIG. 10 is a cross section showing the structure of a conventional liquid crystal display device. FIG. 11 is a perspective depiction showing the structure of electrodes in the conventional liquid crystal display device. FIG. 12 is a plan view showing the state in which a plurality of liquid crystal display panels of the conventional liquid crystal display device are joined.
As illustrated in FIG. 10, the conventional liquid crystal display device is constructed by closely disposing two pieces of liquid crystal display panels 101 and 102 on a flat surface of a piece of a large transparent substrate 102. This liquid crystal display device is a direct-viewing liquid crystal display device incorporating a backlight (not shown) behind the liquid crystal display panels 101 and 102. The backlight and a driver for controlling image signals are omitted in FIG. 10.
Each of the liquid crystal display panels 101 and 102 is constructed by holding a liquid crystal layer 107 between a TFT substrate 110 made of a transparent insulating substrate and a counter substrate 111. Namely, the liquid crystal layer 107 is formed by placing the TFT substrate 110 and the counter substrate 111 to face each other, fastening them with seals 106, and sealing in a liquid crystal therebetween.
Thin film transistors 116 are arranged in a matrix form on the TFT substrate 110. The thin film transistor 116 is usually a field effect transistor using a semiconducting film, and controls the supply of an image signal to a pixel electrode 115. On the other hand, color filters 104 are arranged in a matrix form on the counter substrate 111. The color filters 104 are usually formed by R (red), G (green), B (blue) pixel regions. These pixel regions are separated from each other by a black matrix 105.
When connecting such liquid crystal display panels 101 and 102, it is necessary to fill a refractive index adjusting agent 119 in a joint (joined line) 118. The refractive index adjusting agent 119 prevents diffusion of light due to unevenness between the edges of the liquid crystal display panels 101 and 102 at the joint 118, and renders the joint 118 less noticeable.
The refractive index adjusting agent 109 is required to also function as an adhesive agent when fastening the large transparent substrate 103 and the liquid crystal display panels 101 and 102. It is therefore necessary to use a material having adhesive and sticky properties, for example, an ultraviolet-ray-setting resin used for fastening optical lenses.
Moreover, by placing polarizing plates 108 substantially over the entire front and back surfaces of a large panel which is formed by connecting the above-mentioned two pieces of liquid crystal display panels 101 and 102 with the refractive index adjusting agent 109 so that the polarization axes thereof cross each other at right angles, a liquid crystal display device of multi-panel type is fabricated.
In general, a direct-viewing liquid crystal display device includes a backlight such as a cold-cathode lamp. When a liquid crystal display panel placed in front of the backlight modulates light from the backlight according to image information, the image information input to the liquid crystal display panel is visualized.
At the intersections of the pattern of the seals 106 and the patterns of electrical wiring 113 and 114, the following phenomenon is usually observed. FIG. 13 is an enlarged plan view showing a structure in the vicinity of the seal 106.
The electrical wiring 113 and 114 is formed by patterning a metal film such as Ta, Cr and Al using a photolithography technique. Since the metal film is usually formed with a thickness between 2000 .ANG. and 5000 .ANG., a corresponding gap is produced at the edge sections of the electrical wiring 113 and 114. In this case, as illustrated in FIG. 13, if the seal 106 is applied across the electrical wiring 114 (scanning electrode), the seal 106 gradually oozes out along a gap of 2000 .ANG. to 5000 .ANG. of the electrical wiring 114, and the linearity of the seal pattern is ruined.
Thereafter, when forming the liquid crystal display panels 101 and 102, regions having different cell gaps are produced in the liquid crystal display panels depending on whether the electrical wiring 113 and 114 is present. Consequently, the degree of spread of a sealing agent on a plane is greater in the seal 106 formed in a region with a smaller cell gap (i.e., the intersection of the seal 106 and the electrical wiring) than in the seal 106 formed in a region with a greater cell gap (i.e., a section where no electrical wiring is present), resulting in a disorderly pattern of the seal 106.
In particular, in the case of a liquid crystal display device fabricated by the multi-panel technique, i.e., by connecting a plurality of liquid crystal display panels as described above, it is necessary to form a thin pattern of seal 106 with excellent linearity very close to the pixel electrodes 115. Therefore, the oozing sealing agent and the disorderly seal pattern directly cause a serious problem such as the erosion of the pixel regions 120 by the sealing agent.